While conscious and in the erect position, the lower esophageal sphincter (LES) and laryngeal closing reflex of a healthy person prevent regurgitation and aspiration of stomach contents. Administration of anesthesia and/or sedative/analgesic drugs often compromises many natural reflexes, including these reflexes that help protect one's airway from such regurgitation and aspiration. Upon induction of and emergence from anesthesia, as well as intravenous sedation, a patient is at greatest risk for aspiration of gastric contents, particularly because the patient is usually supine. One study has suggested that as many as 20% of patients given general anesthesia for surgery suffer at least some regurgitation and subsequent aspiration. See, Harris, et al., Can. Anaesth. Soc. J., 31:599 (1984).
More than fifty years ago, C. L. Mendelson thoroughly described, for the first time, the symptoms of aspiration of stomach contents into the lungs, now known generally as aspiration pneumonia. Mendelson, C. L., Am. J. Obstet. Gynecol. 52:191 (1946). Mendelson's observations, which were made of pregnant women anesthetized during labor, led him to describe two types of aspiration: 1) aspiration of solid gastric material, resulting in blockage of the patient's airway and risk of subsequent suffocation and/or pneumonia; and 2) aspiration of liquid gastric material, resulting in pneumonia and/or other asthma-like symptoms. Aspiration pneumonia, also sometimes referred to as Mendelson's Syndrome, was reproduced experimentally by injecting human acidic vomitus into rabbit tracheas. However, injection of neutralized vomitus caused no such symptoms. It is observations such as these that have led researchers to the conclusion that the acidity of the material aspirated from the stomach is the primary causative factor in aspiration pneumonia. See for example, Kinni, et al., J. Oral Maxillofac. Surg. 44:378-384 (1986); and Mendelson, C. L., Am. J. Obstet. Gynecol. 52:191 (1946).
While the fully anesthetized patient is at greatest risk of developing aspiration pneumonia, patients subjected to local anesthesia and/or sedative-analgesic medications are also at risk, particularly as the sedation nears the level of hypnosis. To emphasize the importance of the proper monitoring of patients given sedative and/or analgesic medications prior to diagnostic or therapeutic procedures, the American Society of Anesthesiologists (ASA) has eliminated use of the term "conscious sedation" from its practice guidelines and instead refers to "sedation/analgesia". In those guidelines, the ASA recommends that all patients receiving such medications be monitored by a designated individual who is primarily responsible for administration of the sedative and analgesic drugs and for subsequently monitoring the patient's vital signs, which person should not participate in performing the medical procedure. See "The Changing Role of Monitored Anesthesia Care in the Ambulatory Setting," Rego, et al. Anesth Analg., 85:1020-36 (1997), referring at page 1021 to "Task Force on Sedation and Analgesia by Non-Anesthesiologists. Practice guidelines for sedation and analgesia by non-anesthesiologists: A report by the American Society of Anesthesiologists Task Force on Sedation and Analgesia by Non-Anesthesiologists," Anesthesiology, 84:459-71 (1996). Thus, as used herein, references to the pre-anesthetized patient are intended to refer likewise to those patients scheduled to undergo local anesthesia and/or to be medicated with sedative-analgesic drugs for the purposes of performing a diagnostic and/or therapeutic procedure.
Efforts to eliminate aspiration pneumonia in the anesthetized/sedated patient have focused on both neutralizing the pre-operative stomach contents and reducing the residual gastric volume (RGV) present at induction of anesthesia and/or sedation/analgesia. To this end, guidelines were established for the pre-operative treatment of patients which sought to reduce the risk of aspiration pneumonia by requiring the patient to abstain from all food and all but clear liquids for at least about 12 hours prior to induction and from all liquids for at least eight hours prior to induction. Traditionally, these guidelines have been implemented by a) requiring all patients to fast from midnight the day before their procedure (referred to as nulla per os--nothing by mouth--or the NPO guidelines); b) alkalinization and/or emptying of the stomach just prior to administration of anesthesia/sedation; c) particular vigilance for aspiration during induction of and emergence from anesthesia/sedation; and d) the use of tracheal intubation, during general anesthesia, to protect the airway and lungs from such regurgitation should it occur. These guidelines, though generally used as much as a century ago, still define the standard of care for the pre-operative patient and have seen little variation in all these years of use.
Until about twenty years ago, it was a relatively simple matter to enforce and monitor the NPO guidelines. Patients typically spent at least the night before surgery in the hospital and thus could be denied food and drink after midnight regardless of whether their surgery was scheduled for 8:00 a.m. the next morning or 4:00 p.m. the next afternoon. Further, these patients could be given intravenous hydration and/or glucose loading solutions to help alleviate the dry mouth, hunger, headaches and nausea frequently associated with fasting, not to mention the irritability those symptoms produce. Thus, compliance with these guidelines was rarely, if ever, a problem.
Today, however, most surgeries are performed on an out-patient basis, partly in response to technological and pharmacological advancements and partly in response to economic pressures, such as those brought about by managed care. By the year 2000, it is estimated that 75% of all surgical procedures, more than 37 million operations per year, will be performed on an out-patient basis. This means that increasingly large numbers of patients will be relied upon to follow the pre-operative, NPO guidelines on their own without medical supervision. Prior to surgery, or other diagnostic or therapeutic procedure requiring anesthesia/sedation, each patient is asked whether he/she has fasted as instructed. Not only must the medical provider rely on the patient to honestly answer such questions, but, depending upon how the question is posed, the medical provider must also assume that the patient understood what it was he/she was supposed to not do.
While most patients appear to be compliant with the NPO guidelines, many of them complain of headaches, hunger, thirst, irritability, lightheadedness, and similar symptoms associated with fasting for long periods of time. Additionally, a recent survey of patients at an out-patient facility revealed that although patients were given specific guidelines permitting them to ingest clear liquids up to 4 to 6 hours prior to their scheduled time for surgery, the majority of patients opted to fast from midnight, either out of concern about potential aspiration or because of misinterpretation of the instructions. (Informal survey conducted by the inventor at the Frost Street Out-Patient Surgery Center in San Diego, Calif.) At this particular facility, of the nearly 7,000 patients scheduled annually for surgery, 0.41% (28.7 patients) admitted to violating the NPO guidelines. Because these patients failed to follow the prescribed guidelines their surgeries were cancelled or postponed at significant expense to the facility and inconvenience to the patient, the medical providers and the medical staff. The patients' explanations for breaking the fast included forgetfulness, overwhelming thirst or hunger, and simple misunderstandings of the guidelines such as, what was meant by "clear liquids". In addition to the 0.41% of patients at this facility who admitted to violating the NPO guidelines, it must be assumed that some number of patients violated the guidelines but did not admit to doing so and that still other patients failed to follow the guidelines, but did not realize that they had done so. Thus, the total number of patients that either intentionally or unintentionally violated the NPO guidelines would be expected to be much higher than that actually reported.
These experiences of the Frost Street Out-Patient Surgery Center validate two conclusions of a recent larger scale survey concerning the NPO guidelines: (1) that long fasting prior to elective operation is not only uncomfortable for the patient but has detrimental effects, including causing thirst, hunger, irritability, noncompliance and resentment in adult patients, and (2) that confusion exists among physicians as to which "clear liquids" are appropriate prior to surgery, so it is no wonder that patients are not be able to identify the appropriate beverages. See, Green, et al., Anesth. Anal. 83:123-8 (1996). Thus, it is clear that compliance with the NPO guidelines presents a serious problem with respect to modern day surgical procedures which are preformed primarily on an outpatient basis.
In addition to issues regarding compliance with the NPO guidelines, the suitability of the guidelines for their purpose has recently been questioned. It has generally been accepted among those of skill in the art that a patient having a residual gastric volume of about 25 ml or more (0.4 ml per kg body weight) and/or a gastric pH of about 2.5 or less, at the time of surgery, is at greatest risk of aspiration pneumonia. It is for this reason that the NPO guidelines have strictly required that patients fast for long periods of time prior to induction in order, presumably, to reduce the RGV and to increase or stabilize the pH of the patients' gastric contents. However, studies have shown that as many as 45% of elective surgery patients, despite having fasted as required, have RGV and/or pH values beyond these critical values and are therefore at significant risk for aspiration pneumonia. Hutchinson, et al., Anaesth. Intens. Care, 3:198 (1975); Newson, A. J., Anaesth. Intens. Care, 5:214 (1977), and Coobs, et al., Anesth. Anal., 58:183 (1979). Furthermore, recent studies have suggested that prolonged fasting may actually increase gastric volume and acidity (that is, decrease pH), proving to be harmful to the pre-operative patient rather than protective. Scarr, et al. Can. Med. Assoc. J. 141:1151-4 (1989); Splinter et al., Can. J. Anaesth. 37:36-9 (1990); Sandbar et al., Anesth. 71:327-30 (1989); Hutchinson et al., Can. J. Anaesth., 35:12-5 (1988). In fact, for more than four decades it has been known that normal adults and children, ingesting clear liquid volumes of 150 to 300 ml, are apparently able to empty their stomachs in 10 to 20 minutes, making the 8 hour fasting period normally required by the NPO guidelines seem quite extreme. See, Hunt, J. N., J. of Physiology (London) 132:267-88 (1956).
Despite the forgoing evidence that adherence to the NPO fasting guidelines does not sufficiently reduce either or both of the defined risks of aspiration pneumonia, most hospitals, clinics, and doctors adhere strictly to the guidelines. Thus, what is needed is a new treatment method that will both address the need to reduce the acidity and volume of the stomach contents of the pre-operative patient and address the need to increase patient compliance with the required pre-operative treatment, whatever it may be. Additionally, it is important to bear in mind that doctors, hospitals, clinics, and managed care administrators are frequently slow to accept changed guidelines with respect to the standard of care for patients; therefore, it is additionally important that any new treatment of the pre-operative patient be acceptable to these medical providers.